Why does a bike turn

[Uncle Hud]

(I am out of likes, Fred W.)

 

[Uncle Hud]

^^^^^ out of likes, so +1

 

[NTXFJR]

Great point Mike. Maybe that's why the Oroville dam never got the funding to get properly repaired ahead of time because politicians diverted the funding to black hole studies such as why a bike turns.....

 

[dcarver]

..and back on topic. I have throughly enjoyed reading this thread.. learned a lot. Thought through most of it and still am processing various conflict points. Physics is way coolio.

As for the jump trick, there's definitely not enough speed in the rear wheel of an idling dirt bike to be gyroscopic. And yes, while airborne, they close the throttle.

Usually, but not always. This is going off track a little from how the bike steers, but it does talk to how momentum can affect the bike's attitude. Once a bike has launched off a jump and is airborne it is difficult for a rider to alter its trajectory since there is no ground contact to be used as a fulcrum to lever against.

If an MX rider feels he is going to land with the front wheel too high he will hit the rear brake in mid jump. The momentum of the rear wheel will be transferred into the rear brake caliper and lever the front end down a little. Likewise to raise the front in mid air he'll whack the throttle open. Newtonian inertial physics are what is in play.

So there is some small amount inertial mass to the wheels, both rotational and gyroscopic, but not nearly enough to be of any significance compared to the influence of turning the steering slightly and changing the tire's path on the ground.

I read this whole thing, and there’s some truth here and there throughout, along with quite a bit of stuff that appears to have been made up on the fly because it sounds good. The post by Northwoods Snowman comes closest to painting the whole picture, but doesn’t quite cover the wall on the first coat. So I’m going to take a shot at it.

First, I want to be clear with anyone who bothers to read this on some terms I’m going to use. First there is the center of gravity, or “CG”. That speaks to the point in a 3 dimensional object around which it will rotate if it were to be tossed into the air and spun in any direction, and the point at which it would be balanced end for end, side to side, top to bottom if it could be perched on that single point. It’s found up in the middle of a motorcycle somewhere, and reflects the weight distribution in any of those directions. The “X axis” is the fore and aft center line running through the CG, and when the bike rotates on this axis, it is said to “roll”. The “Y axis” is a vertical line through the CG, around which the bike “yaws”. The “Z axis” runs from left to right through the CG, and the bike “pitches” around this one.

Vehicles with two inline wheels steer, or turn, using a combination of dynamics that blend to varying degrees, very well or poorly, depending on the overall chassis setup. First, since it’s been brought up so many times, let’s examine counter steering. Counter steering can be used to initiate a turn by encouraging the motorcycle, bicycle, or scooter to bank, or lean into a corner. Essentially, it is a way to force the bike to roll on the X axis in spite of the angular momentum (gyro effect) of the wheels which resists its doing so. The concept is rather like that used to balance a hammer or broom on one’s finger tip with the heavy end up. Balance is maintained by shifting the low end of the mass in any direction on the X-Z plane, causing the object to roll or pitch in such a way as to remain upright and counteract the tendency to fall one way or other. When a bike running straight and upright is counter steered to the left, the contact points of the wheels are moved to the left under the CG, and the bike rolls, or falls, toward the right. Note that it is also true that one can simply shift the rider weight to the right over the CG and cause the bike to roll right in response to that without counter steering, so counter steering is not an essential element of turning, just a more effective way of initiating a turn more quickly and with less motion than shifting the rider weight.

Gyroscopic precession does not initiate or maintain a turn at all, and in fact works specifically against any rotation of the axle in any direction from its position on the Z axis. It is a highly stabilizing influence on the chassis, owing to the mass of the rotating tire and rim assembly. The strength of the effect depends the mass involved, and on the diametrical location of the rotating mass, the farther from the center, the more effective it is. It resists leaning the bike in either direction, and that is the main reason that counter steering is used to more forcibly cause the lean than by shifting the weight of the rider.

Someone mentioned bicycles with very small wheels being ridable, which they are, but to anyone who suggests that the gyro effect doesn’t stabilize the bike, I would say they should try to ride a Razor scooter on a pair of the training rollers posted earlier. Such small wheeled bikes are balanced almost entirely through counter steering in the manner of balancing a hammer, as discussed above.

As to the whole tire contact patch thing, there may be some effect in that, but before you buy into that being a primary causal effect, try rolling a coin across a flat surface and watch what happens when it leans one way or other; it turns in that direction, and the more it leans, the tighter the turn is. Why? Think of the diametrical edge of the coin, wheel, etc., not as a continuous line, but as a series of points along the circle. When upright on edge, it rolls straight because, as it rolls off of one point of contact and onto the next, the next point that rolls down onto the ground is coming from a point directly in front of the current contact point, and the next one after that is also directly in front, and so on. When the coin leans left, the next point approaching the ground as the coin rolls is located to the left of vertical, and the next is farther left, and the next farther left than that, so the coin turns in a circle to the left. As the coin leans farther, the circle tightens until it finally looses traction and centripetal force makes it slip and fall. All that with a sharp edge as a contact patch. Any wheel and tire assembly will naturally roll in a curved line in the direction it is leaned over from vertical for this reason alone. The size of the circle will depend on the radius of the tire.

So part of the reason a motorcycle banks into a turn is to cause the wheels to roll in the direction of the lean, but also, there is the matter of countering the centripetal force that would otherwise cause the bike to fall over to the right when turned left. The Y axis through the CG must be placed in a balanced condition against this force in order to counter it.

Then there is the complex matter of “rake”, more accurately called steering axis, or steering head angle, and how it interacts with trail. As the previously posted link showed, trail is the distance between a line drawn through the steering axis and the point at which the tire touches the ground. Normally (and hopefully) this point is behind the intersection of the steering axis and the ground. Trail is the cause of most of the self centering force on the steering while riding in a straight line, caused by the drag of rolling friction pulling back along the X axis at the contact point, but it also has a very pronounced effect on the chassis behavior when the vehicle is banked or rolled from vertical, and this effect works in combination with the steering axis angle to give the bike much of its cornering and steering behavior.

Visualizing how all this ties together is aided by getting your hands on a bicycle, or a motorcycle light enough to be leaned over quite a way without touching the handlebars. If you hold a bike by its seat and lean it to the left, you see that the wheel will “fall in” to point to the left of the X axis. This happens because the force of gravity, which was pushing up in line with the Y axis, is now working at angle to that axis and is able to rotate the steering assembly by pushing against what amounts to an effective lever, called a force arm, that is the length of the trail measurement, turning the wheel toward the direction of lean. In the first few degrees of lean, the “fallover” of the front wheel in this static, non-rolling condition is more pronounced, and the wheel of the bicycle actually begins to turn back more in line with the chassis as it is leaned farther. This happens for two reasons. The first is that the bicycle is standing still, and there is no “caster” or self centering effect on the steering because there is no drag pulling back on the tire from the contact point. Remember this point as we continue.

The second reason the wheel begins to turn back toward straight as lean is increased, and the reason it doesn’t fall all the way 90 degrees to the side in the first place, is because the arc of the leading edge of the tire comes in contact with the ground. What happens here is that the contact patch of the tire has moved from a point behind the steering axis (trail measurement) to a point just about even with it, neutralizing the force of gravity that is trying to turn the wheel in by pushing up and over on the force arm created by trail. In motion, this effect can be problematic. Lots of us have had the experience of a bicycle front wheel suddenly turning sharply inward, or “tucking under” as a result of the wheel falling in too far, and the contact patch getting ahead of the steering axis, and then being launched over the front as we “ride over” the front wheel. At low speeds and with steep head angles, this can happen.

Head angle plays into this such that shallower head angles (more “rake”) reduce the tendency to tuck, and steeper ones encourage it. Picture a really extreme chopper, say with a nearly 90 degree steering axis. With such a setup, steering depends a great deal on the natural circular tracking of a leaned over wheel, and a lot more rotation of the fork in the steering head is necessary to produce the same amount of steering. There is very little tendency to tuck, but a very pronounced tendency to push the front end, or understeer. Conversely, with a steep head angle, the front tire can move the tire contact patch ahead of the steering axis very early in the lean-in maneuver, showing a severe tendency to tuck, but at the same time very little fork rotation is needed to steer the machine, and it will seem very precise and responsive to small steering inputs.

The balance that has to be struck at speed is between the caster, or centering effect of trail, and the tendency for the front wheel to turn in. Head angle has to be chosen based on the amount of stability required versus how responsive the steering should be. If too shallow, the bike will turn in a vague and sluggish manner and tend to push the front, but it will be less prone to speed wobbles. If too steep, it will be skittish and unstable at speed, and prone to wobble, but will turn seemingly with just a thought. Once the head angle is chosen to match the intended use of the bike and other factors, trail is chosen to compliment and modify the turn in behaviors.

So, a turn at speed is initiated most effectively by counter steering to start the bike leaning, at which point the contact patches of both the front and rear tires shift to the side of the tire center lines in and amount governed by the tire size and shape, and the combined influences of the tire’s circular form, steering head angle, and trail produce a more or less balanced natural amount of fork rotation, while centripetal force holds the bike from falling to the inside of the corner. How this complex interaction actually works will affect whether you find yourself applying force to the bars to steer the bike into the corner more, or pulling back toward straight as the corner proceeds.

When it all works right, it feels easy and natural. But it’s definitely not simple.

This makes sense to me. I can visualize the words.

...and the practical side says...



Coolio Thread. Keep going, boys.

 

[Fred W]

For you dorks that are so upset by this ongoing thread, let me introduce you to a forum feature that you will find can improve your life infinitely in the future. They call it the "unsubscribe" button. Use that little gem and you won't be bothered by all the fake science pontification (in that thread anyway), or other blathering internet denizens again.

Epic threads where I have employed this useful tool include:

The Darkside

What did you do to your FJR today (most just rode it to work)

How's the weather where you are

Airplane on a conveyor belt

And many others I have blocked from my memory as well as my email notices.

I can see this thread joining the ranks.

Edited to add: . (In case someone thought I was being serious)

 

[NTXFJR]

Hey DC, <insert smart ass remark here> True the guy in your post makes sense but it's technically off topic. He explains how to induce a bike to turn, but not the dynamic factors involved with why a bike actually turns. Mechanics vs. physics.....

 

[HotRodZilla]

As a resident Dork, I just want to say, I am NOT offended by the livelihood of this thread. I just like giving you guys shit!

However, on the Wonder Woman note, have any of you seen the new one? I went with my daughter's softball club team. Let's just say the dads were very impressed with the new WW. Dang!!

 

[Uncle Hud]

Thanks, Fred W, but contrary to my public howls of boredom, I find this thread amusing. Make that pretty damn funny.

 

[Northwoods Snowman]

I just have to sit here and laugh, and then write that I'm laughing.

 

[TestPilot]

Shouldn't that be ...LEANED a lot - just sayin'

..and back on topic. I have throughly enjoyed reading this thread.. learned a lot. Thought through most of it and still am processing various conflict points. Physics is way coolio.

 

[SacramentoMike]

Beaten to death, when the answer is SO SIMPLE.

A bike turns because the road turns. If the bike didn't turn, you'd hit the freaking trees. Jeez.

 

[wil780]

Recently I was reminded that I had left an issue improperly addressed during a discussion with Fred W. In commenting on a post of mine he wrote:

I do not think any actual significant slipping of the tire tread has anything to do with "normal" turning, right up until you are exceeding the point of tire adhesion (weight / thrust / friction).

To which I replied:

Fred W, I think we agree on the key point. A bike goes around a corner because the front wheel is turned inwards, away from the direction it would point for the bike go in a straight line (assuming something like a line from a helicopter kept it from falling over). That generates the sideways force which deflects the bike from a straight path. What we're quibbling about is whether the tire slips sideways slightly as it generates that sideways force. I see the slip as a consequence of the tire being turned into the corner, and it was a mistake to emphasize the slip instead of the inward turn. Since all of the contact patch except the very centre is already slipping forward or backwards (because of the geometry of the contact patch relative to the axle), I think the tire is operating with sliding friction rather than static friction, so a bit of sideslip is required to generate the turning force. But that's really irrelevant to the issue of why a motorcycle turns, and I never should have brought it up. The front tire generates the turning force because it is turned inwards, away from the direction the bike is moving at any instant.

That was when I was still fishing for an explanation of the tread distortion mechanism of turning (which I eventually figured out for myself). What I wrote wasn't what I really think because the explanation of slip angle that I was using at the time was too complicated to try to describe convincingly and I didn't want to divert the discussion. However, recently I ran across a description of slip angle that is a lot easier to understand, and I thought I should correct what I wrote.

Slip angle is easiest to understand when thinking about how a car turns, since it's a lot simpler than a motorcycle. Consider what happens when you're driving in a steady crosswind. The wheel is turned into the wind, but the car is moving straight ahead. If you look at the position of the front tires when the steering wheel is turned the same amount, it's clear the tire has to be slipping sideways a bit in a crosswind. If you release the wheel you see that this is the case even when the wind-induced turn is very gentle.

The same thing happens in a turn. The wheel is turned to the side from the direction the car is moving, and consequently the tire must be slipping sideways slightly. The tighter the turn, the more turning force you need, so the slip angle is larger (at the same speed). Here is where the mental picture you need for the car gets complicated. If you turn the wheel and move very, very slowly the car will turn with essentially no slip. However, the faster the car moves through the same turn the more turning force is needed, and the wheels point increasingly into the turn from where they would be pointed if you were moving at nearly zero speed, in order to generate the increasing amount of slip that creates the greater turning force at higher speeds.

A motorcycle is even more complicated because the tread distortion mechanism can provide all the turning force needed at small lean angles. Only when more turning force is needed, at moderate lean angles, does the front wheel start to point into the turn and generate a slip angle. (For completeness, I should add that the turning force generated by tread distortion depends critically on the shape of the tire. Choose the wrong shape and it can provide too much turning force. Cossalter's group has documented that it is possible to fit a front tire that makes so much tread distortion force that the tire has to be pointed slightly out of the turn to corner stably at gentle lean angles.)

Finally, if you're interested in this issue, be aware that you won't find any of these ideas in magazine articles and books written by journalists. I've read at least two motorcycle magazines a month for nearly fifty years and two of those books and I have not seen these ideas anywhere except in scientific articles and engineering textbooks. (I haven't read Lee Parks' book, by the way, but I doubt there's anything there that I've missed since no one who has read his book has posted any ideas like these.)

 

[Northwoods Snowman]

Wil780, you need to take the term "slip" and banish it from your memory. Slippage is the absence of friction. The absence of friction can't turn something. When a tire slips you enter into dynamic friction (two surfaces sliding past each other) which is ALWAYS less than static friction (two surfaces staying stationary to each other). You talk about "slip angle", and I like this definition of it: "In vehicle dynamics, slip angle or sideslip angle is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing." In other words if you have a high slip angle, you're going in a different direction than that in which the wheels are pointed. A good example would be the front end of the bike washing out in a sandy corner. That would be a high slip angle; the wheel is turned but the bike is still going straight ahead.

 

[Donal]

I have tried but just never 'grasped' the physics that several contributors have outlined here. I really don't understand it and basically I am not interested.

However, I have tried observing what happens when I am in a cornering situation:

I initiate the lean and entry to the corner by counter steering (pushing on the inside handlebar).

I have really tried to detect a point when I release pressure on the inside handlebar but have never experienced it.

As far as I can detect I maintain pressure on the inside bar while continuing to turn.

If the turn tightens up I just apply more pressure to the inside bar.

If the bend opens up I release pressure.

As the bend straightens up I push on the outside bar to bring the bike back to vertical.

YMMV